1. Field of the Invention
The present invention relates to a lithographic apparatus, device manufacturing method, and an associated device manufactured thereby.
2. Description of the Related Art
Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device may be used to generate a desired circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist).
The term “patterning device” as here employed should be broadly interpreted as referring to a device that can be used to impart an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate; the term “light valve” can also be used in this context. Generally, the pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). Examples of such patterning devices include:                a mask: the concept of a mask (also known as a “reticle”) is well known in lithography, and it includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmission mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. In the case of a mask, the support structure will generally be a mask table/holder/holder, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired;        a programmable mirror array: one example of such a device is a matrix-addressable surface having a visco-elastic control layer and a reflective surface. The basic principle behind such an apparatus is that (for example) addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as non-diffracted light. Using an appropriate filter, the non-diffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind; in this manner, the beam becomes patterned according to the addressing pattern of the matrix-addressable surface. An alternative embodiment of a programmable mirror array employs a matrix arrangement of tiny mirrors, each of which can be individually tilted about an axis by applying a suitable localized electric field, or by employing piezoelectric actuation mechanism. Once again, the mirrors are matrix-addressable, such that addressed mirrors will reflect an incoming radiation beam in a different direction to unaddressed mirrors; in this manner, the reflected beam is patterned according to the addressing pattern of the matrix-addressable mirrors. The required matrix addressing can be performed using suitable electronic means. In both of the situations described here above, the patterning device can comprise one or more programmable mirror arrays. More information on mirror arrays as here referred to can be gleaned, for example, from United States Patents U.S. Pat. No. 5,296,891 and U.S. Pat. No. 5,523,193, and PCT patent applications WO 98/38597 and WO 98/33096, which are incorporated herein by reference. In the case of a programmable mirror array, the support structure may be embodied as a frame or table, for example, which may be fixed or movable as required; and        a programmable LCD array: an example of such a construction is given in United States Patent U.S. Pat. No. 5,229,872, which is incorporated herein by reference. As above, the support structure in this case may be embodied as a frame or table, for example, which may be fixed or movable as required.        
For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table/holder/holder; however, the general principles discussed in such instances should be seen in the broader context of the patterning device as set forth here above.
In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table/holder/holder, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion in one go; such an apparatus is commonly referred to as a wafer stepper.
In an alternative apparatus—commonly referred to as a step-and-scan apparatus—each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the “scanning” direction) while synchronously scanning the substrate table/holder/holder parallel or anti-parallel to this direction. Since, in general, the projection system will have a magnification factor M (generally<1), the speed V at which the substrate table/holder/holder is scanned will be a factor M times that at which the mask table/holder/holder is scanned. More information with regard to lithographic devices as here described can be gleaned, for example, from U.S. Pat. No. 6,046,792, incorporated herein by reference.
In a manufacturing process using a lithographic apparatus, a pattern (e.g. as defined by a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book “Microchip Fabrication: A Practical Guide to Semiconductor Processing”, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4, incorporated herein by reference.
For the sake of simplicity, the projection system may hereinafter be referred to as the “lens”; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a “lens”.
Further, the lithographic apparatus may be of a type having two or more substrate table/holder/holders (and/or two or more mask table/holders). In such “multiple stage” devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Dual stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and WO 98/40791, both incorporated herein by reference.
It will be appreciated that the operational sequence of a lithographic apparatus comprises a projection phase, in which the projection system is active. During the projection phase, a substrate, such as a wafer, is exposed. During the projection phase, a reticle stage carries the patterning device. It should be appreciated that the term “reticle” stage as used herein is simply the stage that carries the patterning device. This can also be synonymously referred to as a “mask stage” or patterning device stage.”
The operational sequence of a lithographic apparatus also comprises an exchange phase. During the exchange phase the patterning device is exchanged. In the exchange phase, a positioning system positions the patterning device relative to the reticle stage. The positioning system comprises a reticle carrier (also referred to as a “mask carrier” or “patterning device carrier”), which carries the patterning device. The positioning can be performed in different ways, for example by measuring and control or by mechanical docking. After the positioning of the patterning device relative to the reticle stage, the reticle stage takes the patterning device over from the reticle carrier, after which the reticle carrier moves away from the reticle stage. A fixing mechanism fixes the patterning device relative to the reticle stage.
In known lithographic projection systems, the patterning device is carried on top of the reticle stage. However, due to recent developments, it has become desirable to locate the patterning device below the reticle stage. This relocation introduces new design problems.
When the patterning device is carried on top of the reticle stage, gravity helps the patterning device to maintain its position relative to the reticle stage. The patterning device generally is placed in a recess of the reticle stage, so that the edges of the recess together with gravity provide the fixing of the patterning device relative to the reticle stage. In the case that the patterning device is carried below the reticle stage, this solution is no longer feasible. Therefore, it has been proposed to fix the patterning device relative to the reticle stage by clamping it to the reticle stage, for example by means of an electrostatic clamp.
Also, the need for interfaces on the patterning device grew stronger, which interfaces could be engaged for handling the patterning device without damaging the sensitive areas of the patterning device. As possible interfaces, brackets at the edges of the patterning device or a frame around the circumference of the patterning device have been proposed, as is described in European Patent Application No. 02251364.2. As an alternative, the patterning device can be provided with handling zones, which are adapted to be contacted by other machine parts.
However, more problems still have to be overcome. For example, on one hand, it is advantageous to use a mechanical docking system for positioning the patterning device relative to the reticle stage, as it is reliable, relatively cheap and easy to manufacture. On the other hand, it turns out that when a mechanical docking system is used, an optimal accuracy of the projected image could not be achieved.
Moreover, it was found that during the transfer of the patterning device from the reticle carrier to the reticle stage, inaccuracies in the positioning of the patterning device relative to the reticle stage occur. In addition, the need to reduce idle time has increased with the new developments.